The disclosure of Japanese Patent Application No. HEI 11-264311 filed on Sep. 17, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a technical field of a parallel robot wherein a plurality of actuators are disposed in parallel. Although the present invention is suited for an industrial robot designed for transportation, processing or machining, it is also widely applicable to other devices such as a driving simulator for a motor vehicle.
2. Description of the Related Art
The industrial robot is classified into a serial robot such as an arm-type robot with a serial linkage mechanism wherein actuators are disposed in series and a parallel robot such as a flight simulator with a parallel linkage mechanism wherein actuators are disposed in parallel. In the parallel robot, unlike the case with the serial robot, there is no bending moment applied to rods constituting the linkage mechanism. That is, only a tensile or compressive axial force is applied to the rods. Thus, the parallel robot is capable of operating at a high speed and a high acceleration and ensuring high rigidity.
Parallel robots having six degrees of freedom (related art I), such as a flight simulator, are disclosed in U.S. Pat. No. 5,333,514 and U.S. Pat. No. 5,175,729 and have already been commercialized. In these six-degree-of-freedom parallel robots, a traveling plate can be translated in all directions along three orthogonal axes and rotated around the three orthogonal axes. In other words, the traveling plate can operate with an extremely high degree of freedom. However, six actuators are required to realize movements with six degrees of freedom in such a six-degree-of-freedom parallel robot. It is impossible to dispense with these actuators. Therefore, there is inevitably a limit imposed on an attempt to cut down the cost. This limit makes the six-degree-of-freedom parallel robot expensive.
It is considered that flight simulators for simulating maneuver of an aircraft need to have six degrees of freedom. However, industrial robots do not always require six degrees of freedom. That is, many industrial robots operate satisfactorily as long as they can be displaced with four degrees of freedom, i.e., can be translated in longitudinal, lateral and vertical directions and rotated in a horizontal plane. A specification with six degrees of freedom is an overburden for industrial robots.
Thus, U.S. Pat. No. 4,976,582 discloses a pseudo-parallel robot capable of moving with the aforementioned four degrees of freedom. This robot will be referred to as xe2x80x9crelated art IIxe2x80x9d to be distinguished from a four-degree-of-freedom parallel robot of the present invention.
The robot of the related art II has three parallel linkages each of which has two rods driven by an actuator. These parallel linkages make it possible to translate a traveling plate with three degrees of freedom. To enable the traveling plate to be turned in a horizontal plane, the robot of the related art II has a freely expandable torque rod which is rotationally driven by a fourth actuator. The torque rod is coupled at one end to a shaft of a rotational motor through a universal joint and at the other end to a central portion of the traveling plate through a universal joint. Therefore, the traveling plate follows rotational movement of the drive shaft of the rotational motor and is rotated in the horizontal plane.
In the robot of the related art II, the number of actuators can be reduced to four. As a result, this robot is less expensive than the aforementioned six-degree-of-freedom parallel robot which requires six actuators.
However, in the robot of the related art II, since a torsional moment is applied to the torque rod, it is difficult to accomplish a movement at a high speed and a high acceleration as intrinsically required of the parallel robot. That is, if a great torque is applied to the torque rod, it undergoes torsional information. For this reason, the robot of the related art II cannot be rotated as precisely as those of the related art I. If the robot of the related art II is stopped from being rotated all of a sudden, it causes torsional vibrations. In this case, it takes a long time to position the robot.
In addition, as described above, since the robot of the related art II has the torque rod susceptible to torsional deformation, its rigidity tends to be insufficient. Therefore, if a yawing moment is applied to the traveling plate, it is rotated in accordance with torsional deformation of the torque rod. As a result, the robot of the related art II is disadvantageous in that the traveling plate cannot easily be positioned with high rigidity and high precision.
It is thus an object of the present invention to provide a four-degree-of-freedom parallel robot capable of displacing a traveling plate with four degrees of freedom at a high speed and a high acceleration and positioning the traveling plate with high rigidity and high precision.
According to a first aspect of the present invention, a four-degree-of-freedom parallel robot comprising a base, four actuator fixed to said base, four rod members each of which is coupled at one end to a movable portion of said actuators through a kinematic element, and a traveling plate whose four corners are coupled to the other ends of said rod members through kinematic elements, wherein at least part of said traveling plate can be displaced with four degrees of freedom which are defined by linear motions along three orthogonal axes and one rotation around a predetermined axis.
In the first aspect of the present invention, the four actuators are controlled in an interconnected manner. If the four rod members are pushed or pulled by predetermined strokes, the traveling plate is displaced by the rod members. The main portion of the traveling plate moves with a limited number of degrees of freedom, i.e., four degrees of freedom. That is, the main portion can be translated in all directions and rotated around a predetermined axis. Therefore, if a position of one end of each of the four rod members on the side of the actuators is determined, a position of the other end of each of the rod members on the side of the traveling plate is also determined. Thus, the position and posture of the traveling plate is determined uniquely.
That is, if strokes of the four actuators are determined, the main portion of the traveling plate is positioned at one point in a three-dimensional space through the four rod members. Also, the posture of the traveling plate is determined by a predetermined rotational position around a predetermined axis. In this process, only a tensile or compressive axial force is applied to each of the rod members. That is, these rod members are free of moment as to torsional deformation.
Therefore, if it is ensured that there is no clearance in kinematic elements and the like for the rod members, the traveling plate is fixed to a predetermined position in the three-dimensional space in a predetermined posture with high rigidity as well as high precision. Further, when the actuators are driven to displace the traveling plate, only an axial force is applied to each of the rod members. That is, the rod members have extremely high rigidity and are thus able to endure a great inertia force. Thus, the traveling plate can be accelerated or decelerated at a high acceleration within the range of driving forces of the actuators. As a result, it becomes possible to perform high-speed operation within the range of driving speeds of the actuators.
Thus, the four-degree-of-freedom parallel robot in accordance with the first aspect of the present invention has the effect of displacing the main portion of the traveling plate with four degrees of freedom at a high speed and a high acceleration and positioning the main portion with high rigidity and high precision. This effect cannot be achieved by the aforementioned robot of the related art II.
In addition, the number of actuators can be reduced in comparison with the aforementioned six-degree-of-freedom parallel robot. Therefore, the parallel robot in accordance with the first aspect of the present invention can be manufactured at a lower cost than the six-degree-of-freedom parallel robot.
According to a second aspect of the present invention, a four-degree-of-freedom parallel robot comprising a base, four actuator rods each of which is fixed at one end to said base through a kinematic element and which can be expanded or contracted to desired length, a traveling plate whose four corners are coupled to the other ends of said actuator rods through kinematic elements, wherein at least part of said traveling plate can be displaced with four degrees of freedom, which are linear motions along three orthogonal axes and one rotation around a predetermined axis.
The second aspect of the present invention achieves substantially the same effect as of the aforementioned first aspect. In addition, there is no actuator fixed to the base, and the actuator rods which serve as the actuators and the rod members of the first aspect are adopted. Thus, the overall structure is simplified, and a reduction in size and weight is made possible. For a similar reason, it also becomes possible to further cut down the cost.
Thus, in addition to the effect of the aforementioned first aspect, the second aspect of the present invention achieves the effect of making further reductions in size, weight and cost possible.